In today's market, flat panel displays, such as liquid crystal displays (LCD), are often enhanced with specialized films. The films may be flexible or rigid. Such films are designed to optimize optical performance, e.g., viewing contrast, increasing brightness, removing glare, enhancing color and enhancing the clarity of the flat panel display or improve display functionality, such as bonding a touch panel to the front surface. The films are typically applied to the viewing side of the display. Application methods involve the use of an adhesive that is optically clear and pressure sensitive for easy bonding directly to the display.
Curable adhesives (e.g., heat or light cured) have been used in applications where substrates require substantial permanency and high strength adherence. Conventional adhesives (e.g., tape, silicone), however, are typically not easy to apply, or provide the benefits of curable adhesives. An adhesive material for application of a film to a base material is described in U.S. Pat. No. 6,139,953. For optical product applications, curable adhesives have been desirable, as they can provide optically clear, strongly adhered laminates (e.g., layered substrates).
To achieve both strength and ease of application, hybrid compositions have been developed that can be used in optical applications. For example, a light curable, polyester based adhesive has been used for plastic glazing applications. In digital video disc (DVD or optical discs) bonding and cathode ray tube (CRT) applications, a liquid adhesive formulation has been used. For bead bonding in making retroreflective articles, a curable polymeric network has been suggested.
Strength and application, however, are not the only criteria that many optical substrates/laminates require. Certain optical products are exposed to harsh environmental conditions, such as heat, UV (solar) light, water, etc. For example, vehicle windshields generally exist in outdoor conditions that submit them to all types of weather. These windshields typically include substrates such as acrylic or polycarbonate, adhered to a solar or infra-red (IR) reflecting film made from a multi-layer optical film (MLOF) (commercially available from 3M Co., St. Paul, Minn.). The materials may become optically obstructed if the adhesion between the layers is damaged or compromised.
Light curable liquid acrylic ester adhesives for glass bonding using low intensity ultraviolet (“UV”) light are known. Such adhesives are useful for glass assembly and repair applications in which high intensity UV light is unavailable or impractical.
A number of fast curing low-yellowing acrylate functional oligomer products are known for use in UV/electron beam (“EB”) curable printing inks and the like. However, such products typically have poor adhesive strength to glass.
It is desirable and often necessary for a viable commercial UV/visible curable adhesive suitable for glass bonding to possess several key properties—e.g., having good adhesive strength, fast tact time, optical clarity and reduced yellowing. An additional key property that is highly desirable in an optical adhesive (in the cured state) targeted for use in display applications is reworkability. With regard to reworkability, one or more events can occur during manufacture, shipping, and/or in use that requires the film and adhesive to be removed easily and cleanly from the display and replaced. Some examples of such events are 1) defects in bonding during application of the specialized film to the display may necessitate on-site repair, 2) damage to an LCD occurring during its use, and 3) a component (e.g., LCD, glass, touch panel) of a device becoming defective after placement in the device. Present commercially available adhesives and associated methods fall short with regard to reworkability and with regard to one or more of the above-mentioned other key properties. The present invention offers a solution towards reworkability in providing an efficient rework method that is cost-effective, semi-automated, safe, and reliable.